GHD was initially developed simultaneously by two independent teams in 2016 [1]}, [2]}, in order to discuss non-equilibrium states of integrable quantum many-body systems. In [2]} it is proposed to adapt the hydrodynamic principles to “generalised thermalisation": the local entropy maximisation with respect to the infinitely many conserved quantities. That is, the hydrodynamic principles still apply to non-chaotic, integrable systems, but local fluid are in so-called Generalised Gibbs Ensembles (GGE) [4]}, instead of Gibbs ensembles. This, in conjunction with an extension to GGE of the celebrated Thermodynamic Bethe Ansatz (TBA) [5]}, [6]}, [7]}, constitute the main building blocks of GHD in the hydrodynamic picture. In [1]}, by contrast, a kinetic picture was taken, where a kinetic equation for stable quasi-particles is derived based on the quasi-particle velocity first proposed in [9]}, with the same results. These works gave rise to an understanding of the non-trivial, large-scale, emergent (hydro)dynamics of quantum gases, chains and field theories [10]}, [11]}, [12]} from microscopic properties. Ultimately this approach also has proven well-suited to describe classical integrable systems [13]}, [14]}, [15]}, [16]}, [17]}, and the GHD equation correctly specialises to the hydrodynamic equation for the classical gas of hard rods, proven rigorously earlier [18]}. The hydrodynamic picture advocated in [2]}, in particular, is very powerful, and has provided a wealth of exact results, including dynamical correlation functions [20]}, [21]}, [22]} and fluctuations [23]}, [24]}. GHD has been extended to include external forces [25]} and other type of space-time varying parameters [26]}, [27]} as well as diffusion [22]}, [29]}, [30]}, and to probe integrability breaking [31]}, [32]}, [33]}, [34]}, [35]}. Importantly, the Euler-scale GHD equations with external force have been confirmed in experiments on cold atomic gases [36]}, [37]}, [38]}. GHD is now an extremely active field of research.
